In general, a compressor, particularly, a reciprocating compressor has no crankshaft for converting a rotating movement to a linear movement, thereby having small friction loss and so having higher efficiency in compression than that of a general compressor.
In case that the reciprocating compressor is used for a refrigerator or an air conditioner, as a stroke voltage inputted to the reciprocating compressor is varied, a compression ratio of the reciprocating compressor can be varied to control cooling capacity. Such a reciprocating compressor according to the related art will now be described with reference to FIG. 1.
FIG. 1 is a block diagram showing a structure of an operation control device of the reciprocating compressor in accordance with the related art.
As shown in FIG. 1, the operation control device of the reciprocating compressor according to the related art includes: a current detector 4 for detecting a current applied to a motor (not shown) of the reciprocating compressor 6; a voltage detector 3 for detecting a voltage applied to the motor; a stroke estimator 5 for estimating a stroke estimation value by using the detected current and voltage and a parameter of the motor; a comparator 1 for comparing the calculated stroke estimation value with a preset stroke reference value and outputting a difference value according to the comparison result; and a stroke controller 2 for controlling the operation (stroke) of the compressor 6 by varying the voltage applied to the motor by controlling a turn-on period of a triac (not shown) connected in series with the motor according to the difference value.
Hereinafter, operations of the operation control device of a reciprocating motor compressor in accordance with the related art will now be described with reference to FIG. 1.
First, the current detector 4 detects the current applied to the motor (not shown) of the reciprocating motor compressor 6 and outputs the detected current value to the stroke estimator 5. The voltage detector 3 detects the voltage applied to the motor and outputs the detected voltage value to the stroke estimator 5.
The stroke estimator 5 operates the stroke estimated value X of the reciprocating compressor by substituting the detected current value, the detected voltage value and the parameters of the motor to following formula 1, and applies the operated stroke estimated value X to the comparator 1.
                    X        =                              1            α                    ⁢                      ∫                                          (                                                      V                    M                                    -                                      R                    ⁢                                                                                  ⁢                    i                                    -                                      L                    ⁢                                          i                      _                                                                      )                            ⁢                              ⅆ                t                                                                        Formula        ⁢                                  ⁢        1            
Here, R represents the resistance of the motor, L represents the inductance of the motor, a represents the motor constant, VM represents the voltage value applied to the motor, and represents the current value applied to the motor of the reciprocating compressor, and    ī    represents the time variation amount of the current applied to the motor. That is,    ī    is a differential value of    ī.
Afterwards, the comparator 1 compares the stroke estimation value with the stroke reference value, and applies the difference value according to the comparison result to the stroke controller 2.
The controller 2 controls the stroke of the compressor 6 by varying the voltage applied to the motor of the compressor 6 on the basis of the difference value. This will now be explained with reference to FIG. 2.
FIG. 2 is a flow chart showing an operation control method of a reciprocating compressor according to the related art.
First, when the stroke estimation value is applied to the comparator 1 by the stroke estimator 5, the comparator 1 compares the stroke estimation value with the pre-determined stroke reference value (S2), and outputs the difference value according to the comparison result to the stroke controller 2.
When the stroke estimation value is smaller than the stroke reference value, the stroke controller 2 increases the voltage applied to the motor (S3), and when the stroke estimation value is larger than the stroke reference value, the stroke controller 2 decreases the voltage applied to the motor (S4), thereby controlling the stroke of the compressor. The voltage is applied to the motor by controlling a turn-on period of a triac (not shown) electrically connected to the motor when increasing or decreasing the voltage applied to the motor,
As the reciprocating compressor is applied with an AC voltage, it reacts sensitively to a supplied AC voltage as well as load variation amounts.
Accordingly, the conventional operation control device of the reciprocating compressor always uses a control unit (triac) because it is sensitive to variation amounts in commercially used AC voltage.
That is, the conventional operation control device of the reciprocating compressor controls the stroke of the piston by using a control unit because AC voltages vary under a normal operation condition as well as under an abnormal operation condition in which the load rapidly varies.